Co-pending and co-assigned U.S. patent application Ser. No. 09/539,707 filed on March 31, 2000, and entitled METHOD AND SYSTEM FOR ESTABLISHING CONTENT-FLEXIBLE CONNECTIONS teaches a technique for establishing an open connection (OP-N), mapped across a communications network. The OP-N connection is “concatenatable”, in that an end user can transport arbitrarily concatenated signal traffic through the OP-N connection. In principle, virtually any combination of concatenated and non-concatenated signals may be used, up to the bandwidth capacity of the OP-N connection. The traffic mixture (i.e., the mix of concatenated and non-concatenated traffic) within the OP-N connection can be selected by the end user to satisfy their requirements, and may be changed by the end user as those requirements change, without requiring re-configuration of the OP-N connection. For example, with an OP-6O connection (i.e. N=60, and the OP-60 therefore has a bandwidth capacity equivalent to an Optical Carrier OC-60 signal) an end user could select a traffic mix of: five STS-12c connections on one day, one OC-48c and 12 (unconcatenated) STS-1 connections on another day, and two STS-24 and two STM-4 connections at some other time. Other traffic combinations are also possible, all at the discretion of the end user, and without intervention from a service provider.
It is expected that, in the future, it may be desirable that the bandwidth capacity of an OP-N connection be greater than that permitted by the bit-rate of a signal carried by any one channel (e.g. a wavelength in a Wave Division Multiplexed—WDM, or Dense Wave Division Multiplexed—DWDM network). For example, it may be desirable to set up an OP-768 connection, which would require a line rate of 39.813 GHz to be carried on a single channel, or higher. Thus an OP-N connection may be defined which incorporates an arbitrary number of channels, each of which carries a lower-rate signal (e.g. an OC-48 at 2.488 GHz, or an OC-192 at 9.953 GHz) through respective different wavelengths multiplexed within a single waveguide (e.g. optical fiber) and/or distributed over two or more waveguides. This arrangement permits signals that require too much bandwidth to be transported within a single channel to be split, or inverse-multiplexed, into multiple data streams that may then be transported through respective channels. However, in order to maintain arbitrary concatenatability within such a multi-channel OP-N connection, it is necessary to maintain precise alignment and/or sequencing of the data streams within their respective channels, so that the high-bandwidth signal can be reassembled at a destination node.
Inverse multiplexing, in which a higher rate signal is distributed among several lower rate signals and then recombined at a destination node, is known in the art. For example, U.S. Pat. No. 6,002,692 (Wills) teaches a system in which a higher rate Synchronous Optical Network (SONET) signal (e.g. an OC-48c at a 2.488 GHz line rate) is inverse multiplexed into multiple Asynchronous Transfer Mode (ATM) cells that are then transported across a switch fabric through respective ports at a lower rate (e.g. 622 MHz). In cases where data of a single SONET frame is carried within two or more ATM cells, each of the cells is provided with a respective sequence number so that the cells can be placed into the correct sequence for reassembling the original SONET frame.
The system of Wills is typical of packet-based inverse-multiplexing methods, in that it requires a significant amount of processing to separate the SONET frame into ATM cell payload; formulate ATM cell headers with assigned sequencing numbers; and then re-sequence the ATM cells prior to reassembly of the SONET frame. Such systems are not easily implemented at multiple gigabits per second line rates. Furthermore, such packet-based methods are not relevant to concatenation of SONET signals, where the lower-rate signals are themselves also SONET signals.
U.S. Pat. No. 5,710,650 (Dugan) teaches a system in which a high data rate OC-192 signal (at a 9.953 GHz line rate) is inverse multiplexed into four lower rate OC-48 signals (at 2.488 GHz line rate) which are transported through respective parallel channels (wavelengths). The lower line rate within each channel provides increased dispersion tolerance, so that longer fiber spans can be used without regeneration of the signals. Misalignment between the OC-48 signals (due to the differing propagation speeds of the four wavelengths) is resolved by processing each of the OC-48 signals in parallel to extract their respective 48 STS-1 signals (each having a 51.840 MHz line rate). These STS-1 signals are then buffered and processed to eliminate any misalignment. Treating the signals in this way dramatically reduces the amount of misalignment which needs to be eliminated (in terms of the number of bits) and so reduces the required length of each realignment buffer. However, the parallel circuits required for independently processing each of the STS-1 signals at the low 51.840 MHz line rate greatly increases the cost of the processing circuitry, and imposes severe restrictions on the available concatenation schemes.
Accordingly, a method and apparatus for aligning data streams, independently of any concatenation scheme, and that is readily extendible across an arbitrary number of channels, remains highly desirable.